Обязательный атрибут не работает

Атрибут required не работает в моей форме. Что я делаю неправильно?

Вот мой код в HTML:

<form id="main-contact-form" method="POST">
    <fieldset class="form-group">
      <input id="name" placeholder="Nome" class="form-control" type="text" tabindex="1" required /></fieldset>
    <fieldset class="form-group">
      <input id="email" placeholder="E-mail" class="form-control" type="email" tabindex="2" required /></fieldset>
    <fieldset class="form-group">
      <input id="phone" placeholder="Numero di telefono" class="form-control" type="tel" tabindex="3" required /></fieldset>
    <fieldset class="form-group">
      <textarea id="message" class="form-control" rows="8" placeholder="Scrivi un messaggio.." tabindex="5" required /></textarea></fieldset>
    <fieldset>
      <button name="submit" class="btn btn-primary" id="submitBtn">Invia Messaggio</button>
    </fieldset>
</form>
<div id="feedback"></div>

Вот код php в теге head:

<script>
$(document).ready(function(){

    $( "#submitBtn" ).click(function( event ) {

        //values
        var name=document.getElementById('name').value;
        var email=document.getElementById('email').value;
        var phone=document.getElementById('phone').value;
        var message=document.getElementById('message').value;
        var dataString = {"name": name, "email":email, "phone": phone, "message":message}

        $.ajax({
            type:"post",
            url:"submitForm.php",
            data: dataString,
            success: function(html) {
                $('#feedback').html(html);
            }
        });
      event.preventDefault();
    });
});
</script>